Typically, as shown in FIG. 1, a wireless communication system 10 comprises elements such as a client terminal or mobile station 12 and base stations 14. Other network devices which may be employed, such as a mobile switching center, are not shown. In some wireless communication systems there may be only one base station and many client terminals while in some other communication systems such as cellular wireless communication systems there are multiple base stations and a large number of client terminals communicating with each base station.
As illustrated, the communication path from the base station (BS) to the client terminal direction is referred to herein as the downlink (DL) and the communication path from the client terminal to the base station direction is referred to herein as the uplink (UL). In some wireless communication systems the client terminal or mobile station (MS) communicates with the BS in both DL and UL directions. For instance, this is the case in cellular telephone systems. In other wireless communication systems the client terminal communicates with the base stations in only one direction, usually the DL. This may occur in applications such as paging.
The base station with which the client terminal is communicating is referred to as the serving base station. In some wireless communication systems the serving base station is normally referred to as the serving cell. While in practice a cell may include one or more base stations, a distinction is not made between a base station and a cell, and such terms may be used interchangeably herein. The base stations that are in the vicinity of the serving base station are called neighbor cell base stations. Similarly, in some wireless communication systems a neighbor base station is normally referred to as a neighbor cell.
Duplexing refers to the ability to provide bidirectional communication in a system, i.e., from base station to client terminals (DL) and from client terminals to base station (UL). There are different methods for providing bidirectional communication. One of the commonly used duplexing methods is Frequency Division Duplexing (FDD). In FDD wireless communication systems, two different frequencies, one for DL and another for UL are used for communication. In FDD wireless communication system, the client terminals may be receiving and transmitting simultaneously.
Another commonly used method is Time Division Duplexing (TDD). In TDD based wireless communication systems, the same exact frequency is used for communication in both DL and UL. In TDD wireless communication systems, the client terminals may be either receiving or transmitting but not both simultaneously. The use of the Radio Frequency (RF) channel for DL and UL may alternate on a periodic basis. For example, in every 5 ms time duration, during the first half, the RF channel may be used for DL and during the second half, the RF channel may be used for UL. In some communication systems the time duration for which the RF channel is used for DL and UL may be adjustable and may be changed dynamically.
Yet another commonly used duplexing method is Half-duplex FDD (H-FDD). In this method, different frequencies are used for DL and UL but the client terminals may not perform receive and transmit operations at the same time. Similar to TDD wireless communication systems, a client terminal using the H-FDD method must periodically switch between DL and UL operation. All three duplexing methods are illustrated in FIG. 2.
In many wireless communication systems, normally the communication between the base station and client terminals is organized into frames as shown in FIG. 3. The frame duration may be different for different communication systems and normally it may be on the order of milliseconds. For a given communication system the frame duration may be fixed. For example, the frame duration may be 10 milliseconds.
In a TDD wireless communication system, a frame may be divided into a DL subframe and a UL subframe. In TDD wireless communication systems, the communication from base station to the client terminal (DL) direction takes place during the DL subframe and the communication from client terminal to network (UL) direction takes place during UL subframe on the same RF channel.
Orthogonal Frequency Division Multiplexing (OFDM) systems typically use a Cyclic Prefix (CP) to combat inter-symbol interference and to maintain the subcarriers orthogonal to each other under a multipath fading propagation environment. The CP is a portion of the sample data that is copied from the tail part of an OFDM symbol to the beginning of the OFDM symbol as shown in FIG. 4. One or more OFDM symbols in sequence as shown in FIG. 4 are referred to herein as an OFDM signal.
In addition to the purposes mentioned above, the CP may often be used for frequency offset estimation at the receiver. Any frequency offset at the receiver relative to the center frequency of the transmitted signal causes the phase of the received signal to change linearly as a function of time. The two parts of an OFDM signal that are identical at the transmitter, i.e., the CP and the tail portion of the OFDM symbol, may undergo different phase change at the receiver due to the frequency offset. Therefore, the frequency offset can be estimated by performing correlation between the CP and the tail portion of an OFDM symbol. The angle of the CP correlation indicates the amount of phase rotation that is accumulated over the duration of an OFDM symbol. This accumulated phase rotation may then used for frequency offset estimation. Let the incoming OFDM signal at a receiver be denoted by z(n) where n is the sample index. As illustrated in FIG. 4, let the length of an OFDM symbol in terms of samples, excluding the CP portion, be denoted by N. Let the length of the CP portion be denoted by L. The CP correlation R(n) at any sample index n may be computed as follows:
                                          R            cp                    ⁡                      (            n            )                          =                                                      1              L                        ⁢                                          ∑                                  l                  =                  0                                                  L                  -                  1                                            ⁢                                                z                  ⁡                                      (                                          n                      -                      l                                        )                                                  ·                                                      z                    *                                    ⁡                                      (                                          n                      -                      l                      -                      N                                        )                                                                                                                    (        1        )            
where z* denotes complex conjugate of z and |•| denotes the absolute value of its argument. Although the CP correlation may be computed for many different sample indices, it is expected to have a large value only for sample indices that correspond to the CP portion of the OFDM symbol. The sample index for the largest CP correlation value over the duration of N+L samples may be considered to be the true OFDM symbol boundary.
In a mobile wireless communication system, a client terminal must continuously maintain its receive and transmit operations in time alignment with that of the serving base station. A client terminal may initially achieve the timing alignment with the serving base station through the cell search procedure. For example, in a 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) wireless communication system, the Primary Synchronization Signal (PSS) is used for initial timing acquisition. However, after completing the cell search procedure, the client terminal may continuously track its timing to remain time aligned with the serving base station as it may move toward or away from the serving base station. The time tracking may also be required due to frequency offset that may be present between the client terminal and the base station.
In OFDM systems, one of the commonly used methods for estimating the timing offset between a client terminal and the base station may be the CP correlation. The position of the largest CP correlation may be used as an indicator of the expected true timing position. Since the largest CP correlation position for an individual OFDM symbol may not be reliable due to noise and interference, the timing positions estimated from a number of OFDM symbols may be filtered before being used for any adjustment in the time tracking.
In wireless communication system deployments where frequency reuse is employed, the signals from several base stations may be superimposed. In some cases, the various base stations may not be time synchronized, i.e., the OFDM symbol boundaries for the different cells may not be time aligned. Even if the OFDM symbol boundaries are time aligned at the base stations, the propagation delays from different base stations to the client terminal may be different and therefore the OFDM symbol timing may not be time aligned at the client terminal receiver. Furthermore, in some wireless communication systems, such as 3GPP LTE or LTE-Advanced wireless communication systems, an option of using different CP lengths exists and the exact CP in use by neighbor base cells may not be known a priori to the client terminal. The overall received signal scenario is illustrated in FIG. 5. Under these types of scenarios, the time tracking based on CP correlation may not be reliable because of interference from neighbor cells. Also, the largest CP correlation position may correspond to the timing of a possibly stronger neighbor cell rather than that of the serving cell. This may lead a client terminal to track neighbor cell timing instead of serving cell timing. This may quickly lead to loss of communication between a client terminal and the serving base station. The problem arises because the CP correlation is common to all the cells that are transmitting OFDM signals; it is not specific to any particular cell. This makes it difficult to differentiate between the CP correlations of signals from two different cells.
The 3GPP LTE wireless communication system supports six different channel bandwidths starting from 1.4 MHz to 20 MHz. The sample rate typically used for the lowest channel bandwidth may be 1.92 Msps whereas for the highest channel bandwidth may be 30.72 Msps. In order to support all the different channel bandwidths, a client terminal may implement its internal logic using the sample clock required for the highest channel bandwidth. This may allow the client terminal to control various internal timing and other events in a unified manner. For example, all the timings in a 3GPP LTE wireless communication system may be derived from the basic clock of 30.72 MHz, which is related to the highest channel bandwidth and the subcarrier spacing. Also, the timing requirements for the uplink transmissions are in units of Ts=1/30.72 Msps≈32.55 ns.
For the case of time tracking, the CP correlation may be performed at the sample rate of the incoming signal which is 30.72 Msps for the case of 20 MHz channel bandwidth. The largest CP correlation position in this case provides the timing position estimate with a resolution of one Ts. As shown in FIG. 6, although the Normal CP duration is the same for both the smallest channel bandwidth and the largest channel bandwidth, the maximum number of samples in that duration for the highest channel bandwidth may be 160. The CP correlation for largest channel bandwidth may provide a reasonably accurate estimate for the timing position. However, for the lowest bandwidth case the maximum number of samples in the CP portion may be 10 and therefore the timing position estimate may not be reliable. The timing position estimates using a CP correlation for a 1.4 MHz channel bandwidth can only be accurate within 16*Ts since the sample rate for 1.4 MHz is 16 times slower than the samples rate for 20 MHz channel bandwidth. This in turn may lead to increased timing error.
When the length of the CP is small, the reliability of the largest CP correlation position may be low. This may be especially true when the signal quality is poor due to fading and interference. For example, in a 3GPP LTE wireless communication system when the channel bandwidth is 1.4 MHz, the CP length in terms of samples is only 9 or 10 samples. The CP correlation over only 9 or 10 samples may produce an unreliable estimate of the true timing position of the client terminal relative to the incoming signal from the base station. Furthermore, the estimated timing error may be generally in units of the sample rate of the incoming signal. For the case of 1.4 MHz bandwidth in a 3GPP LTE wireless communication system, the sample rate may be 1.92 Msps. At this sample rate, each sample duration is 1/1.92 MHz≈0.5 μs. The CP duration in terms of time may be on the order of about 5 μs for Normal CP in the case of a 3GPP LTE wireless communication system. A timing error of a few samples at 1.92 Msps leads to an error on the order of few microseconds which becomes a significant portion of the CP duration. For example, an error of five samples may be equivalent to half of the Normal CP duration. During system design the CP duration is generally chosen based on the need to handle the expected delay spread in a particular application. Therefore, not maintaining accurate timing may lead to a reduced budget being available for handling delay spread in a system. Therefore, it is essential for the client terminal receiver to maintain precise timing with the base station signal and minimize any loss in the delay spread budget of the system and the reduced performance due to lack of accurate synchronization.